Move constant systems into their own libraries.

pkg/compiler/lib/src/constants/constant_system.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart2js;
+library dart2js.constant_system;
+
+import '../dart_types.dart';
+import '../dart2jslib.dart' show Compiler;
+import '../tree/tree.dart' show DartString;
+import 'values.dart';
 
 abstract class Operation {
   String get name;
 }
 
 abstract class UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   ConstantValue fold(ConstantValue constant);
 }
 
@@ skipping 85 matching lines @@
       case ">>":  return shiftRight;
       case "<":   return less;
       case "<=":  return lessEqual;
       case ">":   return greater;
       case ">=":  return greaterEqual;
       case "==":  return equal;
       default:    return null;
     }
   }
 }